The buildup of carbon dioxide (CO2) in the earth's atmosphere is currently causing concern among many scientists and others interested in potentially adverse global climate change, and the primary source of this additional CO2 is the result of the combustion of carbon-containing materials, such as fossil fuels. It is desirable, therefore, to find and develop improved methods of reducing the discharge of CO2 into the atmosphere. One method involves sequestering, or storing, the CO2 to keep it out of the atmosphere. There are many natural, biological processes which effect sequestering, as well as artificial process such as those involving capture and underground storage of CO2 and chemical techniques such as conversion to carbonate mineral forms. In addition, however, CO2 may be electrochemically converted into useful and marketable commodities, including, for example, methane, ethylene, cyclic carbonates, methanol, and formic acid or formate. One CO2 conversion process involves the electrochemical reduction of CO2 (ECRC) to formic acid. Formic acid, for example, finds uses in replacing HCl in steel pickling, traditional tanning of leather, formic acid-based fuel cells, as a storage medium for hydrogen (H2) and carbon monoxide (CO) that are then used as fuels and chemical feedstocks, conversion to sodium formate for airport runway deicing, preservatives and antibacterial agents, as an ingredient in bleaching pulp and paper, and as a precursor for organic synthesis, including pharmaceuticals. ECRC to formate and formic acid, however, exhibits the lowest energy requirements.
The electrochemical CO2 conversion process, however, could benefit from reduced energy requirements and improved catalysts, CO2 fixation, and reactor design as well as better-integrated, more efficient process configurations.
ECRC involves a highly interrelated process. The entire process is based upon the balanced flow of electrons, ions in solution, and gas, as well as current density, and voltage, across the entire electrochemical reactor as well as across the individual reactor elements. CO2 is reduced to end products at a cathode to which electrons are supplied via an external electrical connection by the oxidation of water or other compounds at an anode. The anode and cathode are separated by a selective-ion membrane which allows certain ions to migrate from the anode to the cathode, thus completing the electrical circuit. Thus, changes in one part of the cell can affect other parts. Changes to the anode, for example, can effect changes in VCELL (voltage across the entire reactor), current density, and the efficiency of conversion of CO2 to products. Likewise, different cations and different anions, present in the catholyte and the anolyte, can have different effects on catalytic behavior. For example, when an acid is added to the anolyte, the hydrogen ions (protons, H+) from the anode migrate through the selective ion membrane to the catholyte, effecting the formation of formic acid at the cathode. If, on the other hand, sodium hydroxide (NaOH) is added to the anolyte, the sodium ions move across, forming sodium formate (HCOONa) as the end product. In addition, reactor configuration can impact various other elements of the process, such as electrical resistance, which can impact energy requirements. Runtime life of the entire process can also be an important consideration as shorter runtimes can require more frequent regeneration of catalysts and repair of reactor components. Finally, it is very important, and a measure of the performance of the overall process, to maintain a high Faradaic Efficiency (FE) (expressed as a percentage or as a fraction) throughout the process and over time. As used herein, FE indicates the fraction (or percent) of the total current that passes the electrochemical cell that is used to produce the desired product (e.g., formate). The higher the FE, the better, and the maximum FE is 1.0.
Thus, there is a need for an electrochemical process that offers improved overall engineering and overall performance.